Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
Conventional imagers use photodiodes that are light sensitive electronic elements which convert incident light to either current or voltage. The signal from a matrix of such photodiode elements or pixels creates the image. Photodiodes are typically operated in a “charge depletion” mode. In this mode, a capacitor associated with the photodiode in each pixel circuit is pre-charged to 1V-6V reverse bias (or voltage) before light exposure or image acquisition. For instance, the cathode is at a higher voltage level than the anode (the cathode and anode being the two oppositely charged electrodes in a photodiode). A very low leakage current flow is possible between these two terminals. Leakage current is the flow of charge in the “off” state of the device and is an undesirable effect. In the charge depletion mode, the photo current generated by image information would passively deplete or remove the charges stored in the reverse bias, so that the voltage across the photodiode gradually drops as it absorbs light projected by the incoming image.
In some prior art passive pixel circuits, the readout action may restore the photodiode reverse bias to the pre-exposure level and measure the amount of charges required to restore this bias. In some prior art active pixel circuits, the voltage left on the photodiode is measured at the end of the exposure. The photodiode reverse bias is then restored by a separate reset action.
The above mentioned prior art approaches have at least the following limitations: (1) the photodiode leakage current may also deplete the stored reverse bias, thus introducing a type of noise known as shot noise and dynamic range limits; (2) the photodiode responsivity may change with its bias voltage, which may be depleted with increased signal integration, thus introducing undesirable non-linearity; (3) in many active pixel designs, the accumulated signal charge is represented by the voltage across the photodiode capacitance which is a function of the voltage, thereby introducing undesirable non-linearity; and (4) the bias restoration action of the photodiode capacitance may introduce kTC noise (also known as reset noise).